Purge system

ABSTRACT

A purge system for use with a well intervention system and apparatus is disclosed. The purge system permits well fluids contained in a subsea intervention system to be purged prior to retrieval of the intervention system to the surface. The system comprises a vessel ( 44 ) for storing and deploying wireline tooling, the vessel being exposed to well fluids via the well control package ( 16 ) and first fluid communication means ( 58 ) connected between a purging fluid supply and the well control package, and second fluid communication means ( 60 ) connected between the vessel and the well control package wherein, in use, the purging fluid supply to the vessel via the first fluid communication means displaces fluid from the vessel into a second fluid communication means and into the well control package.

FIELD OF THE INVENTION

The present invention relates to a purge system, and in particular to a purge system for use in, for example, subsea wireline intervention applications and apparatus. The present invention also relates to a subsea intervention system incorporating a purge system, and to a well intervention method.

BACKGROUND TO THE INVENTION

In the oil and gas exploration and production industry many well operations require the use of tools which are deployed (and in some cases operated/controlled) into a well on wireline, such as electrically conducting wireline or non-conducting slickline or the like. Wireline operations may include well intervention procedures such as well logging to establish wellbore and formation conditions of a depleting well, or remedial operations, such as re-perforating and water shut-off, for instance. Numerous tools exist for use in various wireline procedures and it is conventionally the case that a number of these tools are stored on site to be used as required.

The present Applicant has proposed the use of a self-contained well intervention system which can be deployed from a vessel and coupled directly to a wellhead located on the seabed, which offers significant advantages. Such a self-contained well intervention system is disclosed in Applicant's co-pending International Patent Application Publication Nos. WO 2004/065757 and WO 2006/003362. The Applicant's intervention system includes a storage chamber within which a number of intervention and other wireline deployable tools may be located. Each tool may be individually selected and subsequently coupled to a wireline for deployment directly through the wellhead and into the well.

The preferred form of the Applicant's self-contained well intervention system is exposed to well fluid when in operation. Accordingly, consideration must be given as to how to contain and control this well fluid when the intervention system is to be detached from the wellhead and retrieved to the surface. If the well fluid is to be contained within the intervention system and retrieved to surface then suitable equipment must be provided at surface level to safely extract the well fluid from the intervention system, and subsequently store and/or dispose of this. This is particularly complex as the well bore fluid will conventionally contain a proportion of gas and as such the surface level equipment must be capable of safely accommodating the removal and safe disposal of multi-phase well fluids, which can be hazardous in atmospheric conditions. Furthermore, the required additional surface equipment will require a dedicated assignment of plant space which on an offshore rig or drilling/production vessel is at a premium.

Prior art reference WO 01/25593 A1 discloses a subsea lubricator device which may purge fluids from a tool housing and into a production flow line. The lubricator device of the WO 01/25593 A1 reference is required to contain well fluids during wireline operations as wireline extends from surface level and into the lubricator to engage a wireline tool to be deployed in the well. However, in Applicant's self contained well intervention system the wireline is provided entirely at a subsea location and is exposed to well conditions. Accordingly, the Applicant's intervention system does not require use of a lubricator. The device of the WO 01/25593 A1 reference includes a tool housing secured to a Blow Out Preventor (BOP) or Well Control Package (WCP) which in turn is secured to a Christmas tree. The BOP/WCP includes a number of valves for use in fluid control. A bypass line is provided between the tool housing and a connector means which is used to secure the BOP/WCP to the Christmas tree such that the valves in the BOP/WCP may be bypassed. Fluid is displaced from the tool housing though the bypass line and into the connector means. In this way the purged fluid bypasses the BOP/WCP. A separate system is provided to displace fluids from the BOP/WCP.

It is among the objects of the present invention to obviate or at least mitigate the above mentioned and other disadvantages by providing a system for use in purging a fluid from an apparatus adapted to be coupled to a subsea wellhead.

SUMMARY OF THE INVENTION

According to a first aspect of the present invention, there is provided a well intervention system adapted to be coupled to a subsea wellhead assembly including a subsea tree coupled to a wellhead and a well control package coupled to the subsea tree, said system comprising:

a vessel for use in storing and deploying wireline tooling, said vessel adapted to be exposed to well fluids via the well control package when in use;

first fluid communication means adapted to extend between a purging fluid supply and the well control package; and

second fluid communication means adapted to extend between the vessel and the well control package;

wherein, in use, purging fluid supplied to the vessel via the first fluid communication means displaces fluid from the vessel into the second fluid communication means and into the well control package.

The intervention system of the present invention is therefore adapted to simultaneously purge fluid from the vessel and at least a portion of the well control package. The present invention therefore eliminates the requirement to provide separate purging apparatus, or at least minimises the extent to which a separate purging apparatus is required, in order to purge fluid contained within the well control package. Advantageously, once the fluid within the vessel has been displaced, the intervention system and optionally the well control package may be detached from the wellhead assembly and returned to the surface.

Preferably, the system, in use, directs the displaced fluid from the vessel towards a production fluid outlet defined in the subsea tree. Accordingly, the purged fluids may be flowed to surface via a conventional production riser and thus handled using conventional production fluid handling equipment. This arrangement therefore eliminates the requirement to provide additional fluid handling equipment at surface level which is dedicated to handling purged fluids. Preferably, the displaced fluids are directed through a fluid passage within the wellhead assembly to which the vessel is coupled. The fluid passage within the wellhead assembly may be defined by a production fluid bore which communicates with the production fluid outlet of the subsea tree. The fluid passage within the wellhead assembly may be defined by a production fluid bore and a subsea tree crossover flow line, wherein the crossover flow line communicates with the production fluid outlet.

Alternatively, the displaced fluid may be directed into the well. Alternatively further, the displaced fluid may be flowed to surface via a dedicated flow path.

Preferably, the first fluid communication means is adapted to be in fluid communication with a first bore within the well control package, and the second fluid communication means is adapted to be in fluid communication with a second bore within the well control package. The first and second bores may be laterally offset and aligned parallel with each other. Alternatively, the first and second bores may be coaxially aligned wherein the first bore is a central bore of the well control package and the second bore is an annulus. The annulus in the well control package may be defined by an isolation sleeve, preferably extending between the subsea tree and the well control package. Preferably, the isolation sleeve isolates the central bore from the annulus. Preferably, the first bore is aligned with a production bore of the subsea tree of the wellhead assembly, and the second bore is aligned with an annulus bore of said subsea tree.

In one embodiment of the present invention, the purging fluid supply may be a container or vessel located at surface level, and the first fluid communication means may extend from the container at the surface to the vessel. In an alternative embodiment of the present invention, the purging fluid supply may be a container or vessel located subsea, and preferably adjacent or in close proximity to the vessel and wellhead. In this embodiment the purging fluid supply may be mounted on a skid, and advantageously on a Remotely Operated Vehicle (ROV) skid which would permit dynamic positioning and control of the location of the purging fluid supply. Alternatively, the purging fluid supply may be directly or indirectly mounted on the vessel.

Preferably, the supply of purging fluid comprises a known volume of purging fluid, which may be selected to displace the required volume of fluid from the vessel. Advantageously, the purging fluid supply comprises sufficient fluid to achieve at least one, and preferably a plurality of purging operations.

The purging fluid may be glycol or a suitable water and glycol mixture. Advantageously, the glycol assists to prevent or substantially minimise the formation of hydrates. It should be noted that other hydrate inhibiting fluids such as methanol or MEG (Methyl Ethyl Glycol) may alternatively be used.

Preferably, the vessel defines a central bore which in use extends towards the wellhead assembly and which is coaxially aligned with a throughbore of the wellhead assembly when the intervention system is coupled thereto. Advantageously, the throughbore of the wellhead assembly is defined by the production bore of the subsea tree and the central bore of the well control package.

Preferably, the vessel of the intervention system comprises a tool storage chamber. Conveniently, the tool storage chamber comprises a plurality of tool storage clamping means capable of retaining a respective tool in a storage position and selectively moving said tool to a deployment position where the tool may be coupled and decoupled to a wireline connection tool from above. Advantageously, the tool storage chamber defines a portion of the central bore of the vessel. Conveniently, a tool when located in the deployment position by a tool storage clamping means may be substantially aligned with the central bore of the vessel such that said tool will also be substantially aligned with the throughbore of the wellhead assembly. In this way the tool may be readily deployed through the wellhead assembly and into the well bore.

In a preferred embodiment the vessel of the well intervention system further comprises a wireline winch assembly, such as that disclosed in Applicant's co-pending UK Application no. 0419781.0. Advantageously, the winch assembly comprises a winch housing within which a wireline winch drum is located and mounted about the central bore of the vessel. Preferably, the winch assembly is located above the tool storage chamber. Beneficially, the winch housing defines a winch cavity within which the winch drum is located wherein, in use, wireline from the winch drum exits the winch cavity, extends upwardly through a first tube or riser, passes through an upper sheave, and extends downwardly through a second tube or riser and into the central bore of the vessel. Accordingly, the winch cavity, in use, is also exposed to well fluids via the central bore and first and second tubes.

In use, purging fluid may be passed through the vessel central bore, tool chamber and winch cavity to displace well fluid therefrom. Advantageously, a crossover fluid conduit is defined between the central bore of the vessel and the winch cavity such that the purging fluid may flow from the central bore and into the winch cavity to displace well fluids therefrom.

Preferably, the second fluid communication means is adapted to extend between an upper portion of the vessel, above the tool storage chamber and the winch assembly, and the well control package. More preferably, the second fluid communication means is adapted to extend between the upper sheave of the winch assembly and the wellhead assembly.

Preferably, the intervention system further comprises flow isolation means for controlling fluid flow.

Advantageously, the system comprises a primary isolation means for removing communication with the well bore. The primary isolation means may be a sub-surface safety valve (SSSV) located within or below the wellhead. The SSSV is well known in the art.

Preferably, the system comprises well fluid isolation means for selectively preventing the flow of well fluid towards the vessel and through the production fluid outlet of the subsea tree. In one embodiment of the present invention, the well fluid isolation means may comprise a single valve means, such as a ball valve, gate valve, bore plug or the like. The single valve means may be a production master valve. In this embodiment, the single valve means, in use, selectively prevents the flow of well fluid both towards the vessel and through the production fluid outlet of the subsea tree. This particular arrangement is advantageously suitable for use in purging a vessel of an intervention system which is adapted to be coupled to a wellhead assembly incorporating a dual bore subsea tree configuration, which is well known in the art. Such a dual bore subsea tree is conventionally known as a vertical subsea tree.

In an alternative embodiment, the well fluid isolation means may comprise first isolation means for selectively preventing the flow of well fluids towards the vessel, and second isolation means for selectively preventing the flow of well fluids through the production fluid outlet of the subsea tree. Advantageously, the first and second isolation means each may comprise valve means such as a ball valve, gate valve or the like, or may alternatively or additionally comprise a plug or the like. In a preferred embodiment the first isolation means is a production master valve and the second isolation means is a tubing hanger plug. This particular arrangement is advantageously suitable for use for purging a vessel of an intervention system which is adapted to be coupled to a wellhead assembly incorporating a single bore subsea tree configuration, which is well known in the art. Such a single bore subsea tree is conventionally known as a horizontal subsea tree.

The intervention system may comprise a plug pulling and/or setting tool for setting in place and optionally subsequently removing an isolating plug, for example into and from a bore within the wellhead assembly.

In preferred embodiments of the present invention the well fluid isolation means is provided in the subsea tree of the wellhead assembly to which the intervention system is coupled. Advantageously, the well fluid isolation means of the present invention may comprise existing valve means within the subsea tree such that the present invention may advantageously utilise existing wellhead assembly configurations.

Preferably, the system comprises well annulus fluid isolation means adapted to selectively prevent fluid flow either into the annulus from the intervention system or wellhead assembly, or alternatively from the annulus and into the wellhead assembly and/or intervention system. Advantageously, the well annulus fluid isolation means may be provided in the wellhead assembly, and preferably in the subsea tree. Beneficially, the annulus fluid isolation means may comprise existing valve means within the wellhead assembly.

Preferably, the system of the present invention comprises vessel isolation means adapted for use in selectively isolating the vessel of the intervention system from the wellhead assembly to which it is coupled. Advantageously, in use, the vessel isolation means may prevent purging fluid provided to the vessel via the first fluid communication means from bypassing the vessel and flowing towards the wellhead assembly. Additionally, the vessel isolation means, in use, advantageously prevents fluid purged from the vessel from re-entering the vessel via the second fluid communication means. The vessel isolation means may comprise valve means such as a ball valve, gate valve or the like. Advantageously, the vessel isolation means may be positioned within the wellhead assembly. Preferably, the vessel isolation means is positioned within the well control package of the wellhead assembly. Preferably also, the vessel isolation means comprises existing valve means within the wellhead assembly to which the vessel to be purged is coupled. Alternatively, the vessel isolation means may be provided within the vessel to be purged.

Advantageously, the system may further comprise closure means for retaining purging fluid within the vessel once purging has taken place, such that the closure means may be activated when the intervention system is to be separated from the wellhead assembly and returned to surface.

Preferably, the system of the present invention comprises non-return valve means for preventing the return of purged fluid back into or towards the vessel. Advantageously, in one embodiment the non-return valve means is provided in the second fluid communication means.

Advantageously, the first fluid communication means comprises a fluid conduit or umbilical, such as coiled tubing or the like, which extends between the purging fluid supply and the well control package.

Preferably, the second fluid communication means comprises a fluid conduit or umbilical or the like which extends between the vessel to be purged and the well control package.

The well control package may form part of the well intervention system. The well control package preferably includes a plurality of well control valve means. Preferably, the first and second fluid communication means are adapted to be coupled to the well control package at a location below at least one, and preferably all of the well control valve means. This arrangement advantageously permits a greater volume of well fluids to be displaced from the well control package during a single purging operation.

According to a second aspect of the present invention, there is provided a system for use in purging fluid from a vessel adapted to be coupled to a subsea wellhead assembly including a subsea tree coupled to a wellhead and a well control package coupled to the subsea tree, said system comprising:

first fluid communication means adapted to extend between a purging fluid supply and the well control package; and

second fluid communication means adapted to extend between the vessel to be purged and the well control package;

wherein, in use, purging fluid supplied to the well control package via the first fluid communication means displaces fluid from the vessel into the second fluid communication means and into the well control package.

According to a third aspect of the present invention, there is provided a method of purging fluid from a vessel when coupled to a subsea wellhead assembly including a subsea tree coupled to a wellhead and a well control package coupled to the subsea tree, said method comprising the steps of:

providing a first fluid communication means extending between a purging fluid supply and the well control package;

providing a second fluid communication means extending between the vessel and the well control package; and

passing purging fluid from the purging fluid supply to the well control package via the first fluid communication means to displace fluid within the vessel therefrom, wherein the displaced fluid flows from the vessel and back into the well control package via the second fluid communication means.

Accordingly, the method of the present invention simultaneously displaces fluid from the vessel and the well control package.

Preferably, the method further involves the step of flowing the displaced fluids to a production fluid outlet of the subsea tree and subsequently to surface level via a production riser.

Alternatively, the displaced fluid may be directed into the well. Alternatively further, the displaced fluid may be flowed to surface via a dedicated flow path.

Advantageously, the method of the present invention comprises the step of isolating the vessel and the production fluid outlet of the subsea tree from the flow of well fluids through the wellhead assembly. This step may be achieved by closing a sub-surface safety valve (SSSV) and a production master valve within the wellhead and wellhead assembly. In one embodiment the vessel may be isolated from the well head assembly by setting in place a suitable plug such as a tubing hanger plug or the like. Advantageously, where a plug is utilised the method may comprise the further step of re-opening the sub-surface safety valve and exposing the plug to well fluid pressure to ensure adequate sealing integrity of the plug has been achieved. Once sealing integrity of the plug has been verified the SSSV is preferably once again closed.

In one embodiment, the method may further comprise the step of isolating the vessel from annulus fluids through the wellhead assembly. This may be achieved by closing one or more valves within the wellhead assembly.

Preferably, the purging fluid is flowed into the vessel at a rate selected to prevent or at least minimise any mixing between the purging fluid and well fluids within the vessel.

Advantageously, the volume of purging fluid is selected in accordance with the volume of the vessel to be purged to ensure that substantially all well fluids have been removed.

Preferably, the method further involves the step of setting a sealing plug within the wellhead assembly and isolating the purging fluid within the vessel such that the intervention system may be detached from the wellhead and returned to surface, and the wellhead can continue to produce well fluids to surface through the production fluid outlet and via a production marine riser.

According to a fourth aspect of the present invention there is provided a method of performing a well intervention, said method comprising the steps of:

providing a vessel for storing and deploying wireline tooling and coupling said vessel to a subsea wellhead assembly including a subsea tree coupled to a wellhead and a well control package coupled to the subsea tree;

selecting and running wireline tooling into the well from the vessel to perform an intervention operation;

retrieving all wireline tooling from the well and into a stored position within the vessel;

providing a first fluid communication means extending between a purging fluid supply and the well control package;

providing a second fluid communication means extending between the vessel and the well control package;

passing purging fluid from the purging fluid supply to the well control package via the first fluid communication means to displace fluid within the vessel therefrom, wherein the displaced fluid flows from the vessel and into the well control package via the second fluid communication means; and

detaching the vessel from the wellhead assembly and retrieving the vessel to surface.

Preferably, the well control package comprises a plurality of well control valve means and the second fluid communication means is provided to extend between the vessel and the well control package at a position below at least one of the well control valve means.

Advantageously, the vessel is a subsea well intervention system.

According to a fifth aspect of the present invention, there is provided a well intervention system adapted to be coupled to a subsea wellhead assembly, said system comprising:

a vessel for use in storing and deploying wireline tooling, said vessel adapted to be exposed to well fluids via the wellhead assembly when in use;

first fluid communication means adapted to extend between a purging fluid supply and the wellhead assembly; and

second fluid communication means adapted to extend between the vessel and the wellhead assembly;

wherein, in use, purging fluid supplied to the wellhead assembly via the first fluid communication means and displaces fluid from the vessel into the second fluid communication means and back into the wellhead assembly.

Preferably, the wellhead assembly comprises a subsea tree coupled to a wellhead, and a well control package coupled to the subsea tree, wherein the first and second fluid communication means are adapted to be in fluid communication with the well control package.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other aspects of the present invention will now be described, by way of example only, with reference to the accompanying drawings in which:

FIG. 1 is a diagrammatic representation of a well intervention system incorporating a purging system in accordance with an embodiment of the present invention, wherein said intervention system is coupled to a wellhead; and

FIG. 2 is a diagrammatic representation of an alternative wellhead to that shown in FIG. 1.

DETAILED DESCRIPTION OF THE DRAWINGS

Reference is first made to FIG. 1 in which there is shown a subsea intervention system, generally indicated by reference numeral 10, in accordance with an embodiment of the present invention. The intervention system 10 is coupled to a wellhead assembly, generally indicated by reference numeral 12, which comprises a horizontal subsea tree 14 and a well control package 16. The well control package 16 defines a throughbore 17 and includes an upper isolation valve 18 and a lower isolation valve 20 including shear rams 22. It should be noted that numerous additional valves may be provided in the well control package 16, but only valves 18 and 20 are shown in FIG. 1 for clarity. The subsea tree 14 defines a central throughbore 24 in fluid communication with the well bore (not shown), and a production fluid outlet bore 26 in fluid communication with the central throughbore 24. In use, well fluid is driven under pressure from the well bore through the central throughbore 24, through the production fluid outlet bore 26 and subsequently to surface via a production marine riser (not shown). The subsea tree 14 also defines an annulus fluid access bore 27.

As conventionally known in the art, the subsea tree 14 comprises a production fluid valve arrangement including a production master valve 28 and a production wing valve 30. Additionally, the subsea tree 14 comprises an annulus valve arrangement including an annulus master valve 32, an annulus wing valve 34 and an annulus access valve 36.

The subsea tree 14 further comprises a fluid crossover conduit 38 extending between the annulus access bore 27 and the production fluid outlet bore 26. More specifically, the crossover conduit 38 extends between the annulus access bore 27 at a location between the annulus master valve 32 and the annulus wing valve 34, and the production fluid outlet 26 at a location between the production master valve 28 and the production wing valve 30. The crossover conduit 38 is conventionally utilised for controlling excessive pressures which may occur in the annulus.

In accordance with conventional arrangements, the wellhead assembly additionally comprises a sub-surface safety valve (SSSV) 40 located within or below the wellhead below the subsea tree. In use the SSSV is adapted to selectively prevent fluid communication between the well bore and the subsea tree 14.

The intervention system 10 defines a central bore 42 which extends along the entire length of the system 10 towards the well head assembly 12, wherein the central bore 42 is coaxially aligned with throughbore 17 of the well control package 16 and throughbore 24 of the subsea tree 14. The intervention system 10 is therefore exposed to well bore fluid.

Intervention system 10 includes a tool storage chamber 44 which comprises a plurality of tool storage clamping means (not shown for clarity) capable of retaining a respective tool (also not shown for clarity) in a storage position and selectively moving said tool to a deployment position where the tool may be coupled and decoupled to a wireline connection tool (not shown) from above. When located in the deployment position a tool is aligned with the central bore 42 of the system 10 such that the tool may be deployed through the throughbores 17, 24 and into the well bore. The tool storage chamber 44 is also exposed to well bore fluids.

Intervention system 10 also includes a wireline winch assembly 46, such as that disclosed in Applicant's co-pending UK application no. 0419781.0, located above the tool storage chamber. The winch assembly 46 comprises a winch housing 47 which defines a winch cavity 50, within which cavity 50 is located a wireline winch drum 48, shown in broken outline, mounted about the central bore 42. In use, wireline from the winch drum 48 exits the winch cavity 50 of the housing 47, extends upwardly through a first tube 52, passes through an upper sheave 54, and extends downwardly through a second tube 56 and into the central bore 42. Accordingly, the winch cavity 50, in use, is also exposed to well fluids via the central bore 42 and the first and second tubes 52, 56.

As noted above, the intervention system 10 is exposed to well fluid when in operation. Accordingly, any well fluid located within the intervention system 10 must be suitably handled when the intervention system 10 is to be detached from the well head 12 and retrieved to surface. The intervention system 10 thus incorporates a system for purging well fluid therefrom, as discussed in detail below.

The purging system includes a purging fluid supply 57 and first fluid communication means in the form of a conduit 58 extending between the purging fluid supply 57 and the well control package 16 below the upper isolation valve 18 and lower isolation valve 22. The purging fluid supply 57 may be located at surface level or alternatively may be located subsea, adjacent the intervention system 10. The purging system further comprises second fluid communication means in the form of a conduit 60 extending between an upper portion of the intervention system 10, specifically the upper sheave 54, above the winch assembly 46, and the well control package 16 below the upper and lower isolation valves 18, 22. The purging fluid may be glycol or a water/glycol mixture. The first fluid conduit 58, in use, is in fluid communication with the bore 17 of the well control package, and the second fluid conduit 60 is in fluid communication with an annulus 11 within the well control package 16. The annuls 11 is defined between in inner surface of the well control package 16 and an isolation sleeve 13 which extends between the subsea tree 14 and the well control package 16. The annulus 11 is in fluid communication with an annulus bore 15 in the tree 14.

It should be noted that the fluid conduit 58 is adapted to be reconfigured between a purging position, as shown in FIG. 1, and a non-purging position (not shown). Thus, when the intervention system 10 is in normal use the first fluid conduit will be configured in the non-purging or operational position, and reconfigured to the purging position when purging of the system 10 is required. This arrangement prevents well fluids located within the intervention system 10 from venting through the first fluid conduit 58. When located in the non-purging position the first fluid conduit 58 is secured to the intervention system 10 via a first or operational stab-in plate, and when located in the purging position, as shown in FIG. 1, the first fluid conduit 58 is secured to the intervention system 10 via a second or purging stab-in plate.

In use, purging fluid is flowed from the first fluid conduit 58, through the well control package 16 and into the intervention system 10 to displace well fluids therein into the second fluid conduit 60 and towards the wellhead assembly 12, as discussed in more detail below. It should be noted that a crossover conduit 62 and crossover valve 64 is provided between the central bore 42 and the winch cavity 50 such that the purging fluid may flow from the central bore 42 and into the winch cavity 50 to upwardly displace well fluids therefrom. In this way well fluids will be displaced from the winch assembly 46 via both the first and second tubes 52, 56 and the upper sheave 54.

When well fluids are to be purged from the intervention system 10 and the system 10 subsequently returned to surface, the following steps will be followed. Initially, all wireline tools are removed from the well bore and returned to a storage position within the tool storage chamber 44. Following this the sub-surface safety valve (SSSV) 40 is closed to remove communication with the well bore, and using the intervention system 10 a tubing hanger plug 66 is set in place to prevent fluid communication along the throughbore 24. As shown, the tubing hanger plug 66 is set in place above the branch of the production fluid outlet bore 26. The annulus wing valve 27, annulus master valve 32 and production master valve 28 are then closed. The annulus access valve 36 and the production wing valve 30 are configured to be open. A spool valve 68 in the crossover conduit 38 is then opened to vent down the intervention system 10 into the production fluid outlet bore 26. Subsequent to this the SSSV 40 is opened to re-establish communication with the well bore in order to verify sealing integrity of the tubing hanger plug 66, after which the SSSV 40 is again closed to remove communication with the well bore. The upper intervention valve 18 in the well control package 16 is then closed. At this stage a number of barriers are provided between the wellhead 12 and the intervention system 10 and the various valves 28,30,32,34,36,68 in the wellhead are appropriately configured such that purging of the system 10 may now be achieved, as discussed below.

The first fluid conduit 58 is moved from the non-purging position to the purging position, as shown in FIG. 1, and purging fluid is introduced into the system 10. The first fluid conduit 58 is advantageously reconfigured using a Remotely Operated Vehicle (ROV). The purging fluid displaces well fluids upwards, through the tool storage chamber 44 and through the winch assembly 46, including the winch cavity by way of crossover conduit 62 with valve 64 open. The tool storage chamber 44 is formed and adapted to optimise fluid displacement therefrom by removing potential “dead” zones which otherwise may trap well fluids or gas. The well fluids are then displaced through the first and second tubes 52,56, through the upper sheave 54 and into the second fluid conduit 60. The second fluid conduit 60 includes a non-return valve 70 to prevent the displaced fluids from re-entering the intervention system 10. Additionally, the second fluid conduit 60 is of a relatively small diameter to assist in the displacement of gas from the system 10. The displaced fluids then enter the well control package 16, flow downward into the upper portion of the subsea tree 14, through the crossover conduit 38 and into the production fluid outlet bore 26. The displaced fluid may then be flowed to surface via the production marine riser (not shown). As noted above, the fluids are displaced through the well control package 16. Accordingly, this arrangement eliminates the requirement to provide separate purging apparatus to displace fluids from the well control package 16.

Once the required quantity of purging fluid is introduced through the intervention system 10 a tree cap plug (not shown) is set in place and the resultant cavity below the tree cap plug and above the tubing hanger plug 66 is tested for sealing integrity, following which the valves of the subsea tree 14 are returned to their original configurations. An intervention system isolation valve 74 may then be closed and the intervention system 10 detached from the wellhead assembly 12 and subsequently retrieved to surface. The well control package 16 may also be isolated from the subsea tree 14 and subsequently detached therefrom and returned to surface.

Reference is now made to FIG. 2 in which there is shown a vertical subsea tree 80 which may be utilised in place of the horizontal subsea tree 14 of FIG. 1. The well control package 16 and intervention system 10 are shown in FIG. 2 for the purposes of clarity but no further description will be given. The subsea tree 80 in FIG. 2 includes two longitudinal bores, a production bore 82, and an annulus bore 84, and two lateral bores, a production fluid outlet bore 86 and an annulus access bore 88. In this arrangement, the annulus bore 84 is not utilised during purging of the intervention system and as such no further description of this will be given.

The subsea tree 80 comprises a production master valve 90 located below the branch of the production fluid outlet bore 86, a production swab valve 92 located above the branch of bore 86, and a production wing valve 94 located in bore 86. Purging of an intervention system may be achieved in a similar fashion to that shown in FIG. 1 and as such no further description will be given. However, in the embodiment shown in FIG. 2 the production master valve 90 is closed while the production swab valve 92 and production wing valve are opened. In this way fluid may be displaced directly through the production bore 82 and into the production fluid outlet bore 86 and ultimately to surface level via the production marine riser (not shown).

The present invention provides a unique system for purging well fluids contained in a subsea intervention system prior to retrieval of the intervention system to surface. Displacing the well fluids subsea from both the intervention system and the well control package of the wellhead assembly eliminates the requirement to utilise separate purging systems or apparatus. Additionally, directing the purged fluids towards the production outlet of the wellhead eliminates the requirement to maintain additional specialised well fluid handling equipment at surface level which would otherwise be required if the intervention system is retrieved to surface while containing well fluid.

It should be understood that the embodiments described above are merely representative of the present invention and that various modifications may be made thereto without departing from the scope of the present invention. For example, purged fluid may be directed to surface via a dedicated flow path or conduit. Alternatively, purged fluid may be directed into the well. 

1. A well intervention system adapted to be coupled to a subsea wellhead assembly including a subsea tree coupled to a wellhead and a well control package coupled to the subsea tree, said system comprising: a vessel for use in storing and deploying wireline tooling, the vessel being exposed to well fluids via the well control package; and first fluid communication means connected between a purging fluid supply and the well control package; and second fluid communication means connected between the vessel and wherein, in use, the purging fluid supplied to the vessel via the first fluid communication means displaces fluid from the vessel into the second fluid communication means and into the well control package.
 2. A system as claimed in claim 1 wherein the system, in use, directs the displaced fluid from the vessel towards a production fluid outlet defined in the subsea tree.
 3. A system as claimed in claim 1 wherein the displaced fluids are directed through a fluid passage within the wellhead assembly to which the vessel is coupled.
 4. A system as claimed in claim 3 wherein the fluid passage within the wellhead assembly is defined by a production fluid bore which communicates with the production fluid outlet of the subsea tree.
 5. A system as claimed in claim 3 wherein the fluid passage within the wellhead assembly is defined by a production fluid bore and a subsea tree crossover flow line, wherein the crossover flow line communicates with the production fluid outlet.
 6. A system as claimed in claim 3 wherein the displaced fluid is directed into the well.
 7. A system as claimed in clam 3 wherein the displaced fluid is flowed to surface via a dedicated flow path.
 8. A system as claimed in claim 1 wherein the first fluid communication means is adapted to be in fluid communication with a first bore within the well control package, and the second fluid communication means is adapted to be in fluid communication with a second bore within the well control package.
 9. A system as claimed in claim 8 wherein the first and second bores are laterally offset and aligned parallel with each other.
 10. A system as claimed in claim 8 wherein the first and second bores may be coaxially aligned wherein the first bore is a central bore of the well control package and the second bore is an annulus.
 11. A system as claimed in claim 10 wherein the annulus in the well control package is defined by an isolation sleeve extending between the subsea tree and the well control package.
 12. A system as claimed in claim 11 wherein the isolation sleeve isolates the central bore from the annulus.
 13. A system as claimed in claim 10 wherein the first bore is aligned with a production bore of the subsea tree of the wellhead assembly, and the second bore is aligned with an annulus bore of said subsea tree.
 14. A system as claimed in claim 1 wherein the purging fluid supply is a container or vessel located at surface level, and the first fluid communication means may extend from the container at the surface to the vessel.
 15. A system as claimed in claim 1 wherein the purging fluid supply is a container or vessel located subsea, and preferably adjacent or in close proximity to the vessel and wellhead.
 16. A system as claimed in claim 15 wherein the purging fluid supply is a Remotely Operated Vehicle (ROV) skid to permit dynamic positioning and control of the location of the purging fluid supply.
 17. A system as claimed in claim 1 wherein the purging fluid supply is directly or indirectly mounted on the vessel.
 18. A system as claimed in claim 1 wherein the supply of purging fluid comprises a known volume of purging fluid, which is selected to displace the required volume of fluid from the vessel.
 19. A system as claimed in claim 1 wherein the purging fluid is glycol or a water and glycol mixture.
 20. A system as claim in claim 1 wherein the vessel defines a central bore which in use extends towards the wellhead assembly and which is coaxially aligned with a throughbore of the wellhead assembly when the intervention system is coupled thereto.
 21. A system as claimed in claim 1 wherein the vessel of the intervention system comprises a tool storage chamber.
 22. A system as claimed in claim 21 wherein the tool storage chamber comprises a plurality of tool storage clamping means capable of retaining a respective tool in a storage position and selectively moving said tool to a deployment position where the tool is coupled and decoupled to a wireline connection tool from above.
 23. A system as claimed in claim 21 wherein the tool storage chamber defines a portion of the central bore of the vessel.
 24. A system as claimed in claim 21 wherein a tool when located in the deployment position by a tool storage clamping means is substantially aligned with the central bore of the vessel such that said tool is substantially aligned with the throughbore of the wellhead assembly.
 25. A system as claimed in claim 1 wherein the vessel of the well intervention system further comprises a wireline winch assembly comprising a winch housing within which a wireline winch drum is located and mounted about the central bore of the vessel.
 26. A system as claimed in claim 25 wherein the winch assembly is located above the tool storage chamber and the winch housing defines a winch cavity within which the winch drum is located wherein, in use, wireline from the winch drum exits the winch cavity, extends upwardly through a first tube or riser, passes through an upper sheave, and extends downwardly through a second tube or riser and into the central bore of the vessel.
 27. A system as claimed in claim 25 wherein a crossover fluid conduit is defined between the central bore of the vessel and the winch cavity such that the purging fluid may flow from the central bore and into the winch cavity to displace well fluids therefrom.
 28. A system as claimed in claim 25 wherein the second fluid communication means is adapted to extend between an upper portion of the vessel, above the tool storage chamber and the winch assembly, and the well control package.
 29. A system as claimed in claim 28 wherein the second fluid communication means is adapted to extend between the upper sheave of the winch assembly and the wellhead assembly.
 30. A system as claimed in claim 1 wherein the system comprises a primary isolation means for removing communication with the wellbore.
 31. A system as claimed in claim 28 wherein the system comprises well fluid isolation means for selectively preventing the flow of well fluid towards the vessel and through the production fluid outlet of the subsea tree.
 32. A system as claimed in claim 31 wherein the well fluid isolation means is a single valve means, such as a ball valve, gate valve, bore plug or the list which, in use, selectively prevents the flow of well fluid both towards the vessel and through the product fluid outlet of the subsea tree.
 33. A system as claimed in claim 32 wherein the well fluid isolation means comprises first isolation means for selectively preventing the flow of well fluids towards the vessel, and second isolation means for selectively preventing the flow of well fluids through the production fluid out of the subsea tree.
 34. A system as claimed in claim 33 wherein the first isolation means is a production master valve and the second isolation; means is a tubing hanger plug.
 35. A system as claimed in claim 34 wherein the intervention system includes a plug pulling and/or setting tool for setting in place and optionally subsequently removing an isolating plug, for example into and from a bore within the wellhead assembly.
 36. A system as claimed in claim 1 wherein the system comprises well annulus fluid isolation means adapted to selectively prevent fluid flow into the annulus from the intervention system or wellhead assembly or from the annulus and into the wellhead assembly and/or intervention system.
 37. A system as claimed in claim 36 wherein the well annulus fluid isolation means may be provided in the wellhead assembly or in the subsea tree.
 38. A system as claimed in claim 1 wherein the system comprises vessel isolation means for selectively isolating the vessel of the intervention system from the wellhead assembly to which it is coupled.
 39. A system as claimed in claim 38 wherein the vessel isolation means comprises existing valve means within the wellhead assembly to which the vessel to be purged is coupled.
 40. A system as claimed in claim 39 wherein the vessel isolation means is provided within the vessel to be purged.
 41. A system as claimed in claim 1 wherein the system includes closure means for retaining purging fluid within the vessel once purging has taken place, such that the closure means is activated when the intervention system is to be separated from the wellhead assembly and returned to the surface.
 42. A system as claimed in claim 1 wherein the system includes non-return valve means for preventing the return of purged fluid pack into or towards the vessel.
 43. A system as claimed in claim 42 wherein the non-return valve means is provided in the second fluid communication means.
 44. A system as claimed in claim 1 wherein the first fluid communication means comprises a fluid conduit or umbilical, such as coiled tubing, which extends between the purging fluid supply and the well control package.
 45. A system as claimed in claim 1 wherein the second fluid communication means comprises a fluid conduit or umbilical which extends between the vessel to be purged and the well control package.
 46. A system as claimed in claim 1 wherein the well control package forms part of the well intervention system, and well control package having a plurality of well control valve means, and said first and second fluid communication means being adapted to be coupled to the well control package at a location below at least one of the well control valve means.
 47. A system for use in purging fluid from a vessel adapted to be coupled to a subsea wellhead assembly including a subsea tree coupled to a wellhead and a well control package coupled to the subsea tree, said system comprising: first fluid communication means adapted to extend between a purging fluid supply and the well control package; and second fluid communication means adapted to extend between the vessel to be purged and the well control package; wherein, in use, purging fluid supplied to the well control package via the first fluid communication means displaces fluid from the vessel into the second fluid communication means and into the well control package.
 48. A method of purging fluid from a vessel when coupled to a subsea wellhead assembly including a subsea tree coupled to a wellhead and a well control package coupled to the subsea tree, said method comprising the steps of: providing a first fluid communication means extending between a purging fluid supply and the well control package; providing a second fluid communication means extending between the vessel and the well control package; and passing purging fluid from the purging fluid supply to the well control package via the first fluid communication means to displace fluid within the vessel therefrom, wherein the displaced fluid flows from the vessel and back into the well control package via the second fluid communication means.
 49. A method as claimed in claim 48 wherein the method further involves the step of flowing the displaced fluids to a production fluid outlet of the subsea tree and subsequently to surface level via a production riser.
 50. A method as claimed in claim 48 wherein the displaced fluid may be directed into the well or flowed to surface via a dedicated flow path.
 51. A method as claimed in claim 48 wherein this step may be achieved by closing a sub-surface safety vessel (SSSV) and a production master valve within the wellhead and wellhead assembly.
 52. A method as claimed in claim 48 wherein the vessel is isolated from the wellhead assembly by setting in place a suitable plug such as a tubing hanger plug.
 53. A method as claimed in claim 52 including the step of re-opening the sub-surface safety valve and exposing the plug to well fluid pressure to ensure adequate sealing integrity of the plug has been achieved.
 54. A method as claimed in claim 48 including the step of further comprising the step of isolating the vessel from annulus fluids through the wellhead assembly by closing one or more valves within the wellhead assembly.
 55. A method as claimed in claim 48 including the step of the purging fluid is flowed into the vessel at a rate selected to prevent or at least minimise any mixing between the purging fluid and well fluids within the vessel.
 56. A method as claimed in claim 48 including the step of setting a sealing plug within the wellhead assembly and isolating the purging fluid within the vessel such that the intervention system may be detached from the wellhead and returned to surface, and the wellhead can continue to produce well fluids to surface through the production fluid outlet and via a production marine riser.
 57. A method of performing a well intervention, said method comprising the steps of: providing a vessel for storing and deploying wireline tooling and coupling said vessel to a subsea wellhead assembly including a subsea tree coupled to a wellhead and a well control package coupled to the subsea tree; selecting and running wireline tooling into the well from the vessel to perform an intervention operation; retrieving all wireline tooling from the well and into a stored position within the vessel; providing a first fluid communication means extending between a purging fluid supply and the well control package; providing a second fluid communication means extending between the vessel and the well control package; passing purging fluid from the purging fluid supply to the well control package via the first fluid communication means to displace fluid within the vessel therefrom, wherein the displaced fluid flows from the vessel and into the well control package via the second fluid communication means; and detaching the vessel from the wellhead assembly and retrieving the vessel to surface.
 58. A method as claimed in claim 57 wherein the well control package comprises a plurality of well control valve means and the second fluid communication means is provided to extend between the vessel and the well control package at a position below at least one of the well control valve means.
 59. A well intervention system adapted to be coupled to a subsea wellhead assembly, said system comprising: a vessel for use in storing and deploying wireline tooling, said vessel adapted to be exposed to well fluids via the wellhead assembly when in use; first fluid communication means adapted to extend between a purging fluid supply and the wellhead assembly; and second fluid communication means adapted to extend between the vessel and the wellhead assembly; wherein, in use, purging fluid supplied to the wellhead assembly via the first fluid communication means and displaces fluid from the vessel into the second fluid communication means and back into the wellhead assembly.
 60. A method as claimed in claim 59 wherein the wellhead assembly comprises a subsea tree coupled to a wellhead, and a well control package coupled to the subsea tree, wherein the first and second fluid communication means is adapted to be in fluid communication with the well control package. 